This invention relates to an apparatus for separating and recovering metallic zinc from an ash-like solid substance composed of a mixture of metallic zinc, zinc oxide and other zinc containing materials.
Zinc is an easily-oxidizable metal, and hence, a molten zinc bath in a melting furnace or the like has the surface covered with oxides. Further, this surface layer of oxide entangles therein a large amount of metallic zinc and its fluidity is very low. Accordingly, this surface layer of oxide generally is in the form of a hard and thick layer. For this reason, there is customarily adopted an operation of scattering a flux, generally a flux of the chloride type, on the surface of a molten zinc bath, separating and recovering metallic zinc involved in the oxide layer by manual agitation. By this operation, the fluidity of the oxide layer is increased and the majority of metallic zinc involved in the oxide layer is recovered into the zinc bath. However, a considerable amount of metallic zinc is still contained in the oxide layer mainly in the form of fine particles. This amount of residual zinc differs greatly depending on the configuration of the surface state of starting materials to be melted or on the fluxing method. For example, in the case of a starting material having a large surface area such as zinc sheets deposited in the electrolysis, the amount of such residual zinc is as large as 1 to 4% by weight based on the starting material. In case such mixture composed of zinc and its oxide (hereinafter referred to as "dross") is treated in the zinc refining process, considerable processing expenses are required for this treatment. Therefore, various methods have heretofore been practised to squeeze out the metal from the dross and thereby obtain a final product directly from the dross.
The conventional methods comprise charging into a vessel having an inclined bottom face dross scraped out of a melting furnace, agitating the dross manually by means of an iron rod provided with scrape plates and thus squeezing out metallic zinc from the dross. According to this method, just before or during the operation a flux or zinc dust is sometimes added to utilize the heat generated by the oxidation reaction. In this method, however, since agitation is performed manually, no uniform agitation can be attained and further, because of the individual metallic zinc particles having the surface covered with a thin oxide layer, the heat resulting from reaction with the flux can not be utilized effectively. Therefore, the metal recovery ratio is relatively low, for example about 40% by weight. Further, considerable labors are required for practice of this method.
As another conventional method, there can be mentioned a method for obtaining crude zinc by melt-extracting a zinc-containing material such as dross, hard zinc and zinc die-cast alloy scrap with use of a rotary furnace. This method is characterized in that a variety of starting materials including the above-mentioned, ash-like dross, scrap metal and chipped metal masses can be treated. Further, in this method, since a high agitation effect can be attained, there is obtained an advantage that the metal recovery ratio is increased. However, the apparatus used is a rotary furnace having a complicated structure and a large size, and since the ratio of filling of the starting material into the furnace is 10 to 40% at most and a large quantity of heat escapes from the furnace, the furnace should be heated directly or indirectly even when hot dross coming from a zinc melting furnace is treated. Moreover, when the ash-like dross is treated, a considerable amount of dusts is carried over from the furnace with its rotation and metallic zinc is partially oxidized and further, since chlorine compounds are contained in such dusts, they are deposited and fixed in ducts of a dust precipitator, resulting in blocking up of a gas passage.